The present invention relates to a television signal scrambling method and apparatus which employ a scrambler for scrambling a television signal and a descrambler for restoring the scrambled television signal to its original unscrambled form.
Scrambling systems are well known in the art and are often employed in cable and other subscription TV systems for assuring that only paid subscribers receive the television signal transmissions to which they subscribe. A paid subscriber is provided with an authorized descrambler for unscrambling a scrambled television signal so that his television receiver may properly process the unscrambled television signal and allow unimpeded reception. Non-subscribers, who do not have the appropriate descrambling equipment, will only receive the scrambled television signal which cannot be properly processed to produce an acceptable video and/or audio output.
All scrambling systems seek to provode an unintelligible video and/or audio signal to an unauthorized recipient which cannot be easily defeated or unscrambled without the proper descrambling equipment. The greater the sophisication of the scrambling provided, the more difficult it is for unauthorized recipients to replicate authorized descrambling equipment. However, as the sophistication of the scrambling technique increases, so does the cost of the associated scrambler and descrambler circuitry. Moreover, the more complex the circuit, the less reliable it becomes.
Several scrambling systems have been implemented using the concept of suppression of the horizontal or vertical synchronizing signals which are present in a transmitted television signal. In such systems, additional signals must be transmitted from the scrambler to the descrambler to cue the descrambler so that it can reconstruct the suppressed amplitude synchronizing signals. Systems using this approach use the sound carrier or other subcarriers to carry the timing information used to reconstruct the suppressed amplitude synchronizing signals. This requires the provision of additional circuits for generating additional subcarriers or for modulating the sound carrier or chroma sub-carrier which adds increased complexity and cost to the overall scrambling system. In addition, when the sound carrier is used to carry the timing information its level becomes critical and in some instances the scrambling system can become unuseable.
The present invention has been designed to overcome the above problems typically associated with scrambling systems. Accordingly, one object of the invention is the provision of a relatively inexpensive, yet highly reliable, scrambling method and apparatus employing a scrambler which scrambles an incoming television signal by suppressing the amplitude of the horizontal synchronizing signals contained therein. The scrambler also impresses whiter-than-white timing pulses on the scrambled television signal which are used by a descrambler to reconstruct the suppressed amplitude horizontal synchronizing signals thereby unscrambling the television signal. The scrambled television signal and whiter-than-white timing pulses impressed thereon occupy the same signal bandwidth as an unscrambled television signal.
An additional object of the invention is the provision of a scrambling method and apparatus in which the timing pulses required by the descrambler to reconstruct the suppressed amplitude horizontal synchronizing signals are transmitted without requiring use of a sound carrier or other subcarriers.
An additional object of the invention is the provision of a scrambling method and appratus employing a descrambler which is capable of descrambling a scrambled television signal containing suppressed amplitude horizontal synchronizing signals and whiter-than-white reconstruction timing pulses.